Pension Plans and Other Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit costs [Abstract]
Service cost	$ 20.2	$ 7.2	$ 40.4	$ 21.5
Interest cost	26.3	13.1	56.2	39.3
Expected return on plan assets	(27.0)	(13.4)	(59.3)	(40.2)
Prior service cost	0.3	0.1	0.6	0.5
Recognized net actuarial (gain)/loss	8.1	6.7	24.4	20.2
Net periodic benefit costs before regulatory adjustment	27.9	13.7	62.3	41.3
Regulatory adjustment	2.6	3.6	8.4	10.6
Net periodic benefit costs	30.5	17.3	70.7	51.9
Contributions by employer			53.1
Contributions by employer, remainder of fiscal year	66.3		66.3
Pension Plans, Defined Benefit [Member] | Westar Energy Inc [Member]
Components of net periodic benefit costs [Abstract]
Service cost	8.0	7.2	24.1	21.5
Interest cost	12.7	13.1	38.1	39.3
Expected return on plan assets	(13.9)	(13.4)	(41.9)	(40.2)
Prior service cost	0.2	0.1	0.5	0.5
Recognized net actuarial (gain)/loss	8.1	6.7	24.4	20.2
Net periodic benefit costs before regulatory adjustment	15.1	13.7	45.2	41.3
Regulatory adjustment	2.8	3.6	8.4	10.6
Net periodic benefit costs	17.9	17.3	53.6	51.9
Contributions by employer			43.1
Contributions by employer, remainder of fiscal year	4.9		4.9
Pension Plans, Defined Benefit [Member] | KCPL [Member]
Components of net periodic benefit costs [Abstract]
Service cost	9.1	8.1	27.4	24.4
Interest cost	9.4	9.8	28.3	29.4
Expected return on plan assets	(10.6)	(9.5)	(31.7)	(28.6)
Prior service cost	0.2	0.2	0.4	0.4
Recognized net actuarial (gain)/loss	8.4	8.9	25.1	26.7
Net periodic benefit costs before regulatory adjustment	16.5	17.5	49.5	52.3
Regulatory adjustment	0.7	0.2	2.1	4.3
Net periodic benefit costs	17.2	17.7	51.6	56.6
Contributions by employer			23.7
Contributions by employer, remainder of fiscal year	61.4		61.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit costs [Abstract]
Service cost	0.7	0.3	1.5	0.9
Interest cost	2.5	1.4	5.4	4.2
Expected return on plan assets	(2.5)	(1.8)	(6.2)	(5.2)
Prior service cost	0.1	0.1	0.3	0.3
Recognized net actuarial (gain)/loss	(0.1)	(0.2)	(0.4)	(0.6)
Net periodic benefit costs before regulatory adjustment	0.7	(0.2)	0.6	(0.4)
Regulatory adjustment	(0.4)	(0.4)	(1.3)	(1.4)
Net periodic benefit costs	0.3	(0.6)	(0.7)	(1.8)
Contributions by employer, remainder of fiscal year	4.6		4.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Westar Energy Inc [Member]
Components of net periodic benefit costs [Abstract]
Service cost	0.4	0.3	1.0	0.9
Interest cost	1.2	1.4	3.7	4.2
Expected return on plan assets	(1.8)	(1.8)	(5.2)	(5.2)
Prior service cost	0.1	0.1	0.3	0.3
Recognized net actuarial (gain)/loss	(0.1)	(0.2)	(0.4)	(0.6)
Net periodic benefit costs before regulatory adjustment	(0.2)	(0.2)	(0.6)	(0.4)
Regulatory adjustment	(0.4)	(0.4)	(1.3)	(1.4)
Net periodic benefit costs	(0.6)	(0.6)	(1.9)	(1.8)
Other Postretirement Benefit Plans, Defined Benefit [Member] | KCPL [Member]
Components of net periodic benefit costs [Abstract]
Service cost	0.4	0.4	1.1	1.1
Interest cost	0.8	0.9	2.5	2.8
Expected return on plan assets	(0.4)	(0.4)	(1.4)	(1.3)
Prior service cost	0.0	0.0	0.0	0.0
Recognized net actuarial (gain)/loss	0.0	(0.1)	(0.1)	(0.3)
Net periodic benefit costs before regulatory adjustment	0.8	0.8	2.1	2.3
Regulatory adjustment	0.1	(0.1)	(0.2)	0.9
Net periodic benefit costs	0.9	$ 0.7	1.9	$ 3.2
Contributions by employer, remainder of fiscal year	$ 4.6		$ 4.6
Wolf Creek [Member]
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share					94.00%
Wolf Creek [Member] | Westar Energy Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share					47.00%	47.00%
Wolf Creek [Member] | KCPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share					47.00%